Polyester resins derived from terephthalic acid and reactive derivatives thereof, such as dimethyl terephthalate, and alkanediols of from e.g., 1 to 10 inclusive carbon atoms, e.g., ethylene glycol and 1,4-butanediol, as well as related diols, such as 1,6-cyclohexane dimethanol, and mixtures of such resins have been known for some time and have become important constituents in injection moldable compositions. Workpieces molded from such polyester resin compositions, alone, or combined with reinforcements, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction. More recently, blends of such polyester resins with one or more second resins have become of significant commercial interest because such second resins, carefully selected, can greatly improve impact strength, as well as tensile strength, modulus and distortion temperature under load in parts molded from such compositions. By way of illustration, such second resins can comprise minor proportions aromatic polycarbonate resins, as described for example, in Kawase et al, U.S. Pat. No. 3,953,539. Also known are such blends in further combination with other resins, such as polyacrylate resins. See, for example, Cohen and Dieck, U.S. Pat. No. 4,257,937.
All such polycarbonate-polyester resin compositions have, however, a tendency to be unstable in the molten states as evidenced by a change in melting point. Especially troublesome in this respect are polycarbonate resin compositions containing minor proportions of polyester resins. Such compositions seem to undergo a chemical reaction commonly referred to as "jumbling". The "jumbling" reaction may be due to catalyst residues remaining in the polyester resin because some success has been achieved in using organic phosphites as melt stabilizers, see, e.g., Hofrichter, Jr., U.S. Pat. No. 2,650,213; and W. German Offen No. 2,710,729; and as color stabilizers in polycarbonate resins, Fritz et al, U.S. Pat. No. 3,305,520. However, considerable variability in the effectiveness of jumbling suppression has been observed with such phosphites, and more reliable melt stabilizers are needed.
It has now been discovered that two specific inorganic phosphorus reagents, monosodium phosphate and monopotassium phosphate, alone, or in combination, are highly effective to stabilize such polycarbonate-polyester resin compositions containing, predominantly, polycarbonate, especially if the polyester is made with a titanium compound catalyst, e.g., tetra octyl titanate. Monosodium phosphate and monopotassium phosphate, both rather innocuous, mildly acidic reagents, are surprisingly effective as stabilizers for polycarbonate-polyester compositions. The stabilizers can be used in various manners, including prior incorporation as a concentrate in the polycarbonate resin, or in the polyester resin. In addition to the high degree of reliability as stabilizers in such compositions, monosodium phosphate and/or monopotassium phosphate do not detrimentally affect any of the resinous components in the composition, e.g., the polycarbonates, or the polyesters, because the prior art mono ammonium phosphates, as will be shown later, at the same levels degrade the individual polymers. In addition, monosodium phosphate appears to be unique as a stabilizer for polycarbonate-polyester blends comprising predominantly polycarbonate because it is superior to the chemically closely related disodium phosphate, as will be shown later.
Especially difficult to melt stabilize are combinations of polycarbonates and polyester resins with third resins such as polyacrylates, as well as flame retarded blends of polycarbonates and polyesters. Such formulations are rendered reliably melt stable with monosodium phosphate and/or monopotassium phosphate, according to the present invention. In addition to the specific instances noted above, melt stabilization can also be induced in other combinations of polycarbonates with polyesters and other resins, especially those in which an active catalyst was used to prepare one or all of the polymers in the blend.